Look, examine, reflect. You hold capital punishment up as an example. Why? Because of what it teaches. And just what is it that you wish to teach by means of this example? That thou shalt not kill. And how do you teach that “thou shalt not kill”? By killing.

[Voyez, examinez, réfléchissez. Vous tenez à l’exemple [de la peine de mort]. Pourquoi? Pour ce qu’il enseigne. Que voulez-vous enseigner avec votre exemple? Qu’il ne faut pas tuer. Et comment enseignez-vous qu’il ne faut pas tuer? En tuant.]

Victor Hugo (1802–1885) French writer, journalist, human rights activist, politician

Speech (1848-09-15), “Plaidoyer contre la peine de mort [An argument against the death penalty],” Assemblée Constituante, Paris
    (Source)

(Source (French)).

I have been unable to find the origin of the English translation that is widely used for this quotation (let alone an English record of the entire speech).